The subject matter disclosed herein relates to electrochemical cells, and in particular an intermediate module for an electrochemical cell stack.
Electrochemical cells are commonly used in a stack configuration for a variety of applications such as electricity generation from hydrogen or hydrocarbon fuel, production and compression of hydrogen, production and compression of oxygen or oxygen-enriched air, or production nitrogen-enriched air. Although stack configurations can vary, a common design involves a series of planar membrane electrode assemblies (MEA), each disposed in a stackable frame, separated by electrically conductive separator plates, also referred to as bi-polar plates. The bi-polar plates serve to connect the stacked MEA's in series, and to separate the fluid on the anode side of each MEA from the fluid on the cathode side of the adjacent MEA in the stack. Fluid flow channels to deliver and receive fluid flow from cells are commonly incorporated in the frames of the stacked components. The stack typically has an end plate at each end of the stack. The stacked components are assembled under a compressive load from bolts extending between the end plates through the stack. However, it can be difficult to maintain this compressive load throughout multiple pressure cycles encountered during operation of the electrochemical cell stack or at higher operating pressures (e.g., ≥100 psi). Commonly used responses to these problems include the use of compression springs in with the nut and bolt connections, however, such techniques only reduce and do not eliminate the problem. Moreover, high endplate loads required to ensure positive cell stack sealing at operating pressures can also induce creep in the cell membrane and other plastic components subjected to the compressive load. In addition, electrochemical cell stacks can be subject to experience reduced electrical conductivity at higher internal pressures due to cell component off-loading caused by the pressure differences between the anode and cathode sides of the stack.
In some applications for electrochemical cells, a process fluid having electrical conductivity is in contact with one or more portions of the MEA's in the stack. For example, some PEM electrochemical cell stacks use water containing an electrolyte such as hydrogen chloride in contact with both sides of the MEA units in the stack, with fluid connections in the stackable frame between adjacent cells. However, differences in electronegative potential between different portions of the stack in contact with the process liquid can lead to shunt currents forming in the process liquid. Such shunt currents can reduce efficiency of the electrochemical cell stack, cause corrosion, and lead to unwanted gas forming from electrochemical reactions in the process liquid (e.g., the generation of unwanted H2, O2, and Cl2 gas in an HCl solution process fluid).
In addition to managing axial loading of an electrochemical cell stack as described above (i.e., loadings along an axis of the stack transverse to the plane of the stacked planar modules), lateral loadings can also be an issue, particularly in the case of higher operating pressures such as encountered with applications such as with the use of electrochemical cell stacks for hydrogen compression. Outwardly-directed lateral stress during operation of the stack can cause deformation of key components, fluid leaks, and electrical anomalies.